Quantitative Spectroscopy and Reaction Kinetics of Reactive Species in Plasma Using Mid-Infrared Optical Frequency Comb Spectroscopy (SKPlaF)

• Applicant: Ibrahim Sadiek, Ph.D.
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 499280974

Our capability of modeling, optimizing and controlling plasma-assisted processes strongly depends on the knowledge of the absolute concentrations and temperatures of reactive species in the plasma and their reaction kinetics. Plasma-assisted nitrocarburizing (PNC) as an eco-friendly alternative to conventional gas nitrocarburizing, is one of the growing industries to improve hardness, and fatigue strength of metals through the diffusion of the reactive nitrogen and carbon into metal surfaces. There is a great demand for such modified components, e.g., in automotive, and aerospace industries. So far, PNC has been mainly investigated by laser absorption spectroscopy (LAS) based on continuous wave (cw) lasers. In principle, LAS is non-invasive and provides absolute concentrations on selected absorption lines. However, the narrow spectral band of the used cw lasers limits the number of detectable molecules, hence representing a hurdle to determine the full plasma composition. This hurdle can be overcome by using optical frequency combs as a light source. With a frequency comb, broadband detection schemes can detect concentrations of tens of process-relevant species simultaneously and hence enable the elementary kinetics to be elucidated.The project aims to develop and test two direct frequency comb based detection methods in the mid-infrared region for high-sensitive multispecies detection in PNC. This involves (i) a fast-scanning Fourier Transform Spectrometer (FTS), providing a full mapping of the absorptions of predominant C-H and N-H containing species (e.g., CH4, C2H2, HCN, and NH3) within the comb bandwidth, and (ii) a so-called Virtually Imaged Phased Array (VIPA) spectrometer, providing a time resolution in the microsecond scale for kinetic measurements of transient radicals (e.g., CH3, CH2, CH, NH, and NH2). A further development of the VIPA spectrometer involves the implementation of an optical cavity to enhance the absorption sensitivity by, at least, four orders of magnitude, which is very essential for detection of the radicals. In addition, the project aims to elucidate the complex hydrogen cyanide, HCN, chemistry, as it is a major molecule in different plasmas involving carbon, nitrogen and hydrogen in the precursors. Particular focus will be on the possible isomerization to the more reactive isocyanide, HNC, isomer (i.e., HCN⇌HNC), which is presumed to react with several species in the plasma, and hence determine its characteristics. The measurement of concentration profiles during PNC under varied conditions of feed gas and plasma power, and a comparison with a chemical kinetic model will serve as a critical test to postulate and validate the so far lacking [H,C,N] elementary mechanisms. In perspective, the methods developed in this project will not be limited to the investigation of plasmas, but can be adapted to other fields, e.g., aerosol chemistry, where the complex gas matrix can benefit from the broadband of frequency comb.

• DFG Programme: Research Grants